A drive torque of a vehicle such as an automobile is generated by a prime mover such as an engine and transmitted through a gearing mechanism such as a clutch and a transmission to wheels. The torque to be inputted from the prime mover can be transmitted to the output side such as driving wheels, if a transmission torque capacity of the gearing mechanism is sufficiently large. However, if the transmission torque capacity is too large more than necessary, a power to be consumed for that purpose is also increased, so that the mileage of the vehicle as a whole is deteriorated. Generally in the prior art, therefore, an oil pressure for setting the transmission torque capacity of the gearing mechanism is set so as to correspond to an output of the prime mover in advance. Otherwise, the control system is constructed to reflect the output of the prime mover on a regulation level of the oil pressure.
Especially, in a continuously variable transmission for a vehicle, if a clamping pressure for clumping a belt, a power roller and so on is raised, the transmission torque capacity is increased, and on the contrary, the transmission efficiency of the power is degraded in the continuously variable transmission. On the other hand, it is necessary to certainly prevent the damage such as a wear caused by a slip, therefore, a high precision is required for the control of the clamping pressure. However, it is not necessarily the case that a running state or a driving state of the vehicle is always constant. Therefore, a high torque acts temporarily on the gearing mechanism such as the continuously variable transmission, and as a result, the slip may occur. Also, there is a case of causing a slight slip intentionally, with the purpose of determining a slip limit pressure for causing the slip.
In the prior art, JP-A-6-11022 discloses a system for detecting the slip on the basis of a comparison result between a theoretical changing rate of the gear change and an actual changing rate of the gear change when the slip occurs in a belt type continuously variable transmission as an example of the gearing mechanism. The system disclosed in JP-A-6-11022 is constructed to suppress the slip in the belt type continuously variable transmission by raising the clamping pressure with increasing a line pressure, and by lowering the output of the engine with closing a throttle opening, delaying an ignition timing, or reducing a feeding amount of a fuel.
As disclosed in the aforementioned JP-A-6-11022, if the clamping pressure is raised to suppress or converge the slip in case the slip is detected in the belt type continuously variable transmission, the torque limited by the slip of the belt to act on the continuously variable transmission is raised. As a result of this, an output shaft torque is varied. Moreover, the torque acting on the continuously variable transmission is lowered when the output of the engine or an electric motor of the input side of the continuously variable transmission, so that the slip can be suppressed or converged. However, if the output of the engine or the electric motor is lowered as the vehicle is running and such a lowering state continues even after said convergence of the slip, a driving torque of the driving wheel is also lowered. Thus, when the output shaft torque or the driving torque varies, an accompanying shock occurs and an uncomfortable feeling may be felt.